Cryogenic cooling apparatus

ABSTRACT

Cooling apparatus includes a generally tubular heat exchanger affording two paths through one of which refrigerant gas from a supply under pressure flows from one, warm, end of the heat exchanger, which in the normal position of use is uppermost, to the other, cold, end and there through a Joule Thomson expansion nozzle, some of the gas being liquefied in a container while thhe remainder returns through the other path, a valve member cooperating with the nozzle to vary its effective area, and a sensor which is located in the container and arranged so that as it comes into heat exchange relationship with liquid refrigerant, it will cause the valve to reduce the effective area of the expansion nozzle. To enable the apparatus to operate effectively in an inclined or inverted position or an accelerated condition the end wall of the container is in heat exchange relationship with a load and the nozzle is directed towards the said end wall to direct the refrigerant to impinge on it, and the sensor has a surface inclined at a small angle to a side wall of the container and spaced from it so as to leave between them a capillary gap tapering to a minimum, in which a liquid drop will be retained in any orientation and the area of the sensor in contact with the liquid will vary widely with the size of the drop. A shield is provided between the sensor and the stream of refrigerant spray from the nozzle to the end wall.

United States Patent [19] Campbell CRYOGENIC COOLING APPARATUS [75] Inventor: David Neil Campbell, Alcester,

Primary Examiner-Meyer Perlin Attorney, Agent, or Firm-Watson, Cole, Grindle & Watson o oWoXoXo) EoXoXo o June 25, 1974 l 5 ABSTRACT Cooling apparatus includes a generally tubular heat exchanger affording two paths through one of which refrigerant gas from a supply under pressure flows from one, warm, end of the heat exchanger, which in the normal position of use is uppermost, to the other, cold, end and there through a Joule Thomson expansion nozzle, some of the gas being liquefied in a container while thhe remainder returns through the other path, a valve member co-operating with the nozzle to vary its effective area, and a sensor which is located in the container and arranged so that as it comes into heat exchange relationship with liquid refrigerant, it will cause the valve to reduce the effective area of the expansion nozzle. To enable the apparatus to operate effectively in an inclined or inverted position or an accelerated condition the end wall of the container is in heat exchange relationship with a load and the nozzle is directed towards the said end wall to direct the refrigerant to impinge on it, and the sensor has a surface inclined at a small angle to a side wall of the container and spaced from it so as to leave between them a capillary gap tapering to a minimum, in which a liquid drop will be retained in any orientation and the area of the sensor in contact with the liquid will vary widely with the size of the drop. A shield is provided between the sensor and the stream of refrigerant spray from the nozzle to the end wall.

6 Claims, 2 Drawing Figures CRYOGENIC COOLING APPARATUS This invention relates to cryogenic cooling apparatus of the type in which the cooling is produced by expansion, through a nozzle, of a refrigerant in the form of gas under pressure, which before expansion is at a temperature below its'inversion temperature, part of the refrigerant being thereby liquefied.

The invention is concerned with cooling apparatus including agenerally tubular heat exchanger affording two paths through one of which'refrigerant gas from a supply under pressure flows from one, warm, end of the heat exchanger, which in the normal position of use is uppermost, to the other, cold, end and there through a Joule Thomson expansion nozzle, some of the gas being liquefied in a container while the remainder returns through the other path, a valve member co-operating with the nozzle to vary its effective area, and a sensor which is located in the container and arranged so that as it comes into heat exchanger relationship with liquid refrigerant, it will cause the valve to reduce the effective area of the expansion nozzle.

It'will be appreciated that the term nozzle is used herein to cover any suitable static device permitting expansion of gas, whether it be a plain orifice, a specially shaped nozzle, or a number of orifices whether alone or associated with a porous plug or membrane, for example as described in the present applicants British Pat. No. 863,961.

Apparatus of the type specified is described for example in U.S. Pat. Nos. 3,517,525, 3,630,047, 3,704,598 and 3,704,601, all commonly owned herewith.

Arrangements such as those described in the prior patents referred to are primarily intended for operation in what may be termed a normal upright position, although they may operate tolerably well when inverted or accelerated. An object of the present invention is to provide an arrangement for requirements in which operation in an inclined or even inverted position or an accelerated condition is of primary importance.

According to the present invention the sensor has a surface inclined at a small angle to a wall of the container and spaced from it so as to leave between them a capillary gap tapering to a minimum, in which a liquid drop will be retained in any orientation and the area of the sensor in contact with the liquid will vary widely with the size of the drop.

The liquid retained by surface tension between the sensor and the container wall is referred to herein as a drop although it will normally be formed partly by a film of liquid creeping across the wall and partly by a protuberance of the liquid film drawn out from the film by surface tension with the sensor acting in the manner of a nib or drawing pen.

In a conventional layout the end wall of the container is in heat exchange relationship with a load and the nozzle is directed towards the said end wall to direct the refrigerant to impinge on it due to the momentum of the liquid aided by vapour flow, and the sensor is located adjacent a side wall laterally spaced from the stream of refrigerant from the nozzle to the load.

A shield may be provided between the sensor and the stream of refrigerant spray from the nozzle to the end wall, the shield preferably extending closer to the end wall than the sensor. This further ensures that droplets of refrigerant will not impinge on the sensor without first impinging on the end wall.

The nozzle and valve should also be formed so as not to obstruct direct projection of droplets on to the end wall. Thus the nozzle may be controlled by, and form a seating for, a valve comprising a needle carried by a support which extends laterally from it and is thin, i.e., having in a direction transverse to the flow of refrigerant a dimension small in relation to its other dimensions, so as to minimise obstruction to the ballistic flow of refrigerant spray from the nozzle to the load.

Nor should the distance from the nozzle to the end wall be excessive, for example it preferably does not substantially exceed 1 cm.

The invention may be put into practice in various ways, but one specific embodiment will be described by way of example with reference to the accompanying drawings in which:

FIG. 1 is a sectional elevation of a cooling apparatus working on the Joule Thomson principle, and

FIG. 2 is a sectional plan view.

Briefly, the apparatus comprises a generally tubular heat exchanger mounted in a container such as av Dewar flask. The heat exchanger is afforded by a helically coiled finned tube through which high pressure refrigerant gas flows from a supply under pressure from what may be termed the warm end of the heat exchanger, to the other cold end, where it emerges through an expansion orifice. The expansion orifice forms the seating for a needle valve controlled by a bellows subjected to the vapor pressure of a suitable liquid in equilibrium with its vapor in a control cavity. Part of the expanded refrigerant liquefies due to the Joule Thomson cooling effect, while the remainder returns through the other path of the heat exchanger, between the fins of the helical tubular coil, at substantially atmospheric pressure so as to cool the incoming refrigerant. The device serves to cool a load such as a radiation detector on the end wall of the container.

Such a cooling apparatus would normally be used with its axis vertical and with its cold end at the bottom, but as it is an important feature of the present invention that the apparatus can be used in an inverted position it is shown in the drawing in an inverted position, i.e., with the cold end upwards.

The apparatus includes a tubular heat exchanger comprising an inner tubular body 10 around which is helically wound a finned inlet tube 11 forming the inlet path of the heat exchanger. An external coaxial tube 12, preferably, as shown, the inner wall of a Dewar flask 13, is located round the finned coil 11 and the space between the inner body 10 and the external tube 12 provides the second or exhaust path of the heat exchanger for exhaust gas flowing past the fins to cool the incoming high pressure refrigerant within the helical coiled tube 11 forming the inlet path.

The Dewar flask 13 has a flat end with an infra-red detector 14 secured to the inner wall 12 in the vacuum space and constituting the load of the cooler.

The lower end of the helical finned tube 11 communicates with a central bore (not shown) in the lower end of the body to which working fluid under pressure is'supplied at a temperature below its inversion temperature. At its upper end the inner tubular body 10 has welded to it a reinforcing ring 16 having a sensor 17 projecting from one point of it, as described below, towards the cold end. Projecting parallel to the axis from a diametrically opposite point of the ring is a seating member 24 of hollow cylindrical form.

The upper end of the helical tube 11 opens into the lower end of the seating member which has in it a filter (not shown) while its upper end has in it an orifice 29 which forms an expansion nozzle for the refrigerant and also provides a valve seating.

Thus the effective area of the expansion nozzle 29 is arranged to be controlled by means of a needle valve 34 which is itself controlled by a bellows 35.

The bellows 35 has its upper open end secured to a sleeve 36, which in turn is secured to the reinforcing ring 16, while its movable closed lower end is secured to the lower end of an operating tube 37. This extends up through the sleeve 36 and at its end carries a shield 38 which is curved so as partly to encircle the sensor as shown in FIG. 2. Welded to the upper end of the shield is a valve carrier 39 which carries the needle valve 34 which has an upper cylindrical portion and a lower tapered portion projecting into the expansion orifice 29 of the seating member 24-.

The sensor 17 is in the form of metal tube 18 sealed in a hole extending through the reinforcing ring 16 parallel to the axis and having its upper end closed. The

sensor tube 17, and the space outside the bellows 35 inside the tubular body 12 of the heat exchanger, form a control chamber filled with liquid and vapor in equilibrium of a suitable material, which may or may not be the same as the refrigerant.

Thus in operation, as described in the prior patents referred to above, as the liquid refrigerant progressively cools the sensor 17, the pressure applied to the outside of the bellows 35 falls correspondingly, and the bellows expands, raising the operating tube 37 and causing the needle valve 34 to progressively close the expansion orifice 29 so as to reduce the flow of refrigerant.

The prior arrangements referred to have employed as a sensing device a hollow probe dipping into a pool of liquid refrigerant in the container, so that as the liquid level rises, a progressive change in temperature gradient occurs in the probe and this gives a progressive reduction in vapor pressure applied to the bellows, so that as the level of the pool of liquid refrigerant rises the valve progressively closes and reduces the flow of refrigerant so as to cut down the amount of cooling produced.

When such a cooler is operated in random attitudes, for example is inclined or inverted or subjected to acceleration, the concept of a liquid level is no longer valid, and it is necessary to direct the jet of refrigerant towards the load. The liquid will then flow over the wall of the container generally back towards the heat exchanger in a thin film.

The arrangement in accordance with the invention is designed to detect this film and control the cooler in accordance with its thickness and abundance.

For this purpose the sensor 17 has a closed end 18 situated close to the side wall of the cylindrical container, whence the tube diverges from the wall at a small angle, preferably between and say, 10.

Accordingly, in operation, if a liquid film creeps across the container wall adjacent the sensor and comes into contact with the sensor, the latter will form a drop by surface tension in the manner of a nib or drawing pen. Initially the area of contact between the drop and the sensor will be a minimum, but as the thickness of the film increases, or indeed the general abundance of liquid spray in the container increases, the size of the drop will increase and its area of contact with the sensor will increase progressively until the apparent liquid level as seen by the probe is sufficient to operate the control mechanism of the cooler, reducing the rate of liquid production by the cooler, and hence reducing the availability of liquid to the probe. Heat influx due to the probe temperature gradient continuously boils off the liquid between the probe and the wall but, due to the tapering gap, the capillary forces ensure that further liquid is drawn into the gap and a control balance is reached when the abundance of supply of liquid to the probe is just sufficient to balance the boil off of liquid at the probe.

The end of the probe is separated by a very narrow gap from the wall or even may be in contact with it. In one specific embodiment in which the container is of cylindrical form with a flat end and has a diameter between 5 and 10 mm, the minimum gap is a fraction of a millimetre.

It will be appreciated that the invention is not restricted to the embodiment specifically described. Thus the probe or sensor need not be of tubular form to its end but as described in British Pat. No. 1,230,079 referred to above, may have a solid end or extended tail so as to give a progressively increasing heat transfer to the liquid and vapour within it. Again the probe may be of bent or curved form so that the gap increases in each of two opposite directions, or its surface presented to the wall may be of very fiat conical or convex shape so that the gap increases from a minimum in all directions.

What I claim as my invention and desire to secure by Letters Patent is:

l. A cooling apparatus including a generally tubular heat exchanger affording two paths through one of which refrigerant gas from a supply under pressure flows from one, warm, end of the heat exchanger, which in the normal position of use is uppermost, to the other, cold, end and there through a Joule Thomson expansion nozzle, some of the gas being liquefied in a container while the remainder returns through the other path, a valve member co-operating with the nozzle to vary its effective area, and a sensor which is located in the container and arranged so that as it comes into heat exchange relationship with liquid refrigerant, it will cause the valve to reduce the effective area of the expansion nozzle, in which the sensor has a surface inclined at a small angle to a wall of the container and spaced from it so as to leave between them a capillary gap tapering to a minimum, in which a liquid drop will be retained in any orientation and the area of the sensor in contact with the liquid will vary widely with the size of the drop.

2. Apparatus as claimed in claim 1 in which the end wall of the container is in heat exchange relationship with a load and the nozzle is directed towards the said end wall to direct the refrigerant to impinge on it due to the momentum of the liquid aided by vapour flow, and the sensor is located adjacent a side wall laterally spaced from the stream of refrigerant from the nozzle to the load.

3. Apparatus as claimed in claim 2 including a shield between the sensor and the stream of refrigerant spray from the nozzle to the end wall.

in relation to its other dimensions, so as to minimise obstruction to the ballistic flow of refrigerant spray from the nozzle to the load.

6. Apparatus as claimed in claim 2 in which the distance of the nozzle from the end wall does not substantially exceed 1 cm. 

1. A cooling apparatus including a generally tubular heat exchanger affording two paths through one of which refrigerant gas from a supply under pressure flows from one, warm, end of the heat exchanger, which in the normal position of use is uppermost, to the other, cold, end and there through a Joule Thomson expansion nozzle, some of the gas being liquefied in a container while the remainder returns through the other path, a valve member co-operating with the nozzle to vary its effective area, and a sensor which is located in the container and arranged so that as it comes into heat exchange relationship with liquid refrigerant, it will cause the valve to reduce the effective area of the expansion nozzle, in which the sensor has a surface inclined at a small angle to a wall of the container and spaced from it so as to leave between them a capillary gap tapering to a minimum, in which a liquid drop will be retained in any orientation and the area of the sensor in contact with the liquid will vary widely with the size of the drop.
 2. Apparatus as claimed in claim 1 in which the end wall of the container is in heat exchange relationship with a load and the nozzle is directed towards the said end wall to direct the refrigerant to impinge on it due to the momentum of the liquid aided by vapour flow, and the sensor is located adjacent a side wall laterally spaced from the stream of refrigerant from the nozzle to the load.
 3. Apparatus as claimed in claim 2 including a shield between the sensor and the stream of refrigerant spray from the nozzle to the end wall.
 4. Apparatus as claimed in claim 3 in which the shield extends closer to the end wall than the sensor.
 5. Apparatus as claimed in claim 2 in which the nozzle is controlled by, and forms a seating for, a valve comprising a needle carried by a support which extends laterally from it and is thin, i.e., having, in a direction transverse to The flow of refrigerant, a dimension small in relation to its other dimensions, so as to minimise obstruction to the ballistic flow of refrigerant spray from the nozzle to the load.
 6. Apparatus as claimed in claim 2 in which the distance of the nozzle from the end wall does not substantially exceed 1 cm. 